File > Demux > Demux to m2v and AIFF...
Or, that's what I see anyway. I'm running MPEG Streamclip 1.9 (with QuickTime Pro, but I'm not sure if that makes a difference).
Use the generated AIFF file to add your audio back to your movie in Final Cut.
To maintain quality, keep it in the format it was captured in. Final Cut should render it internally to DV anyway.
HTH
SL